Ebola: what is it and how does it spread?

Thousands of people in West Africa have died in the worst Ebola outbreak on record, and the World Health Organisation warns the disease could infect 20,000 more before the end of the year.

The deadly viral infection is raging in Guinea, Liberia and Sierra Leone - and has also spread into Nigeria and Senegal. An outbreak said to be unrelated to the one in West Africa has been reported in Democratic Republic of Congo.

There have also been isolated Ebola cases in the United States and Spain.

So what is Ebola, and how does the deadly disease spread?

First discovered in 1976, the virus has periodically spread through parts of Africa, killing thousands in the process.

There is currently no vaccine, and due to its fast onset and horrific symptoms it has become one of the world's most feared diseases.

How does it spread?

There are five strains of Ebola: Zaire, Sudan, Tai Forest, Bundibugyo and Reston. The Zaire strain, which is involved in the latest outbreak, is the most lethal with a fatality rate of up to 90 per cent.

Humans can catch the virus from animals through close contact with infected animals' blood, secretions, organs or other bodily fluids.

The virus is thought to reside within the region's fruit bat population, with the bats carriers of Ebola, but unaffected by it.

The bushmeat trade (the catching and eating of wild animals, including primates such as gorillas and chimpanzees), is thought to play a role in outbreaks of the disease.

While cooking infected meat kills the virus, handling of the meat beforehand can cause infection.

Once in the human population, the virus continues spreading through direct contact with blood, secretions, organs or other bodily fluids.

The World Health Organisation has specifically noted traditional healing and burial practices in rural regions as a factor in the spread of the disease.

The US Centres for Disease Control says people can only catch Ebola by: coming into direct contact with the blood or body fluids of a person who is sick or has died from Ebola; touching contaminated objects; or touching or eating infected animals.

It notes Ebola does not spread through the air or through water.

How does it affect the body?

Why is containing Ebola proving difficult?

In West Africa, the man-made elements of conflict, confusion and culture have all combined to create a perfect-storm for Ebola.

The scientist who first identified Ebola in 1976 gives direct and simple advice on how to contain this latest outbreak:

"Soap, gloves, isolating patients, not reusing needles and quarantining the contacts of those who are ill - in theory it should be very easy to contain Ebola," Dr Peter Piot told the BBC.

In practice, this is a much tougher proposition. The main outbreak has emerged in war ravaged West Africa, where much of the health care infrastructure has been totally destroyed.

Poverty has combined with fear, ignorance and superstition, particularly in remote communities, where distrust of government is understandably high, and belief in witchcraft and sorcery is interwoven into everyday life.

In Liberia, some communities believe the outbreak is a hoax, and that health care workers have been sent to kill them. In one town, health care workers spraying chlorine – a cheap and effective counter to the spread of the disease - were attacked.

In Guinea, Medicines Sans Frontiers (MSF) doctors and medics were attacked by villagers who believed the clinical team had brought Ebola to their country.

An un-coordinated rush by the international community to assist can also complicate efforts, says African governance expert Kim Yi Dionne, especially when it appears that no one is in charge.

Already involved in the Ebola response are the local ministries of health for Liberia, Guinea and Sierra Leone, the World Health Organisation, MSF, UNICEF and many other agencies.

March 22 2014

The current Ebola outbreak was first registered in February in Guinea and is already the deadliest in the history of the disease.From there it has spread to Liberia and Sierra Leone, and then to Nigeria and most recently to the United States.As of December 29, 20,206 people had contracted Ebola in the West African outbreak, resulting in 7,905 deaths (a mortality rate of 39 per cent).Aid organisations are warning the outbreak cannot be contained by the current response, and the virus is likely to reach Europe by the end of October.Scroll along the timeline to see how the outbreak has progressed.

March 22 2014

Guinea's Ministry of Health confirms a mysterious disease that has killed up to 59 people is Ebola.The fatality rate is almost 70 per cent, and four health workers are among the victims.The outbreak originated in the densely forested areas of south-eastern Guinea, but as of March 24, three suspected cases are being investigated in the capital of Conakry. The WHO announces reports of suspected Ebola cases in the border areas of Liberia and Sierra Leone.

March 31 2014

Liberia's Ministry of Health confirms two cases of Ebola among the 27 patients in the country suspected of carrying the disease.In Guinea 122 likely cases of the disease are reported, resulting in 80 deaths there.The WHO reports that the bodies of two probable victims of the disease in Guinea were repatriated back to Sierra Leone.At this time the WHO does not recommend trade or travel restrictions be placed on Liberia, Guinea or Sierra Leone.

April 14 2014

Guinea reports 197 clinical cases of the disease, including 122 deaths, with a growing amount coming from the capital, Conakry.The risk to health workers is also increasing; our of 24 infected in efforts to treat the disease, 13 die.In Mali, several deaths that had been feared to have been caused by Ebola test negative, easing fears the outbreak has spread to that country.In Sierra Leone, 15 people are under surveillance by authorities, but no confirmed cases of the disease have been found.

May 12 2014

The outbreak appears to have stabilised in the region.In Guinea, five out of the six prefectures where the outbreak occurred have no reported any more cases for almost a month.In Liberia no new cases have been reported since April 9, and the WHO predicts that if this continues the outbreak in the country can be declared over by May 22.No confirmed cases have been reported in Sierra Leone.

May 27 2014

On May 23 the WHO is notified of an outbreak of Ebola in Sierra Leone, with four deaths reported.The cases come from a region that borders the area in Guinea that has been a hotspot for the outbreak.By May 27 that number has risen to 16 cases and five deaths.Across the border in Guinea the number of infections is on the rise again, with 14 new cases taking the total toll to 186 deaths out of 281 infections.In Liberia there are no new cases reported.

June 17 2014

The number of infections and deaths has risen rapidly in Sierra Leone to 97, with a fatality rate of over 50 per cent.In Guinea, the disease again begins to gain momentum with cases rising to 398. Of these, 264 have died.In Liberia, where the number of infections had previously stabilised, nine more new cases are reported, resulting in five more deaths. The death of a traditional healer in Sierra Leone highlights the dangers of dealing with disease; 35 confirmed cases are traced back to her funeral when grieving patients handled her body.The healer had previously travelled across the border to Guinea to treat someone infected with Ebola.

July 2 2014

The disease has continued its spread through rural and urban populations in Guinea, Sierra Leone, and Liberia, with Medecins Sans Frontieres (MSF) warning the disease is out of control and will not be stopped by the current response. As of the start of the month 467 people in total have died from the disease, with 769 cases reported.The WHO convenes an emergency meeting with health ministers from 11 African countries to address the spreading epidemic.It highlights the three major factors contributing to the epidemic: traditional beliefs and cultural practices facilitating the spread in rural areas, transmission through densely populated areas such as Conakry and Monrovia, and cross-border transmission through commercial social activities between the borders of the three countries.

July 14 2014

While the levels of infection appear to be stabilising in Guinea, they show no signs of abating in Liberia and Sierra Leone.

July 23 2014

Patrick Sawyer, an American who was working for the Liberian government, collapses on arrival at Lagos Airport in Nigeria on July 20.He is put in isolation at the First Consultants Hospital in Obalende, one of the most crowded parts of Lagos, a city that is home to 21 million people.It later emerges his sister had died from Ebola. Mr Sawyer dies on July 25, prompting an evacuation of the hospital.Before he collapsed he took flights from Liberia to Ghana, on to Togo, and eventually to Nigeria, highlighting the risks international travel poses to the spread of the disease.From the time of his arrival Mr Sawyer was in contact with at least 59 people. They are being monitored by Nigerian authorities.

July 30 2014

Sheik Umar Khan, the doctor leading Sierra Leone's fight against Ebola, dies from the virus on July 29. Mr Khan’s death follows the deaths of dozens of local health workers and the infection of two American medics in neighbouring Liberia, highlighting the dangers faced by those trying to halt the disease's spread.Airlines begin suspending flights to Sierra Leone and Liberia.

August 9, 2014

Hundreds of troops are deployed in Sierra Leone and Liberia to fight the Ebola outbreak.With healthcare systems in the West Africa nations overrun, the World Bank announces up to $200 million in emergency assistance to Sierra Leone, Liberia and Guinea as part of an international effort to contain the virus.More than 60 doctors have already died of Ebola, hampering efforts to control the outbreak.Panic among local communities, which have attacked health workers and threatened to burn down isolation wards, prompted Sierra Leone, Liberia and Guinea to impose the tough measures, including the closure of schools and the quarantine of the remote forest region hardest hit by the disease.In Nigeria, where US citizen Patrick Sawyer died of Ebola in late July after arriving from Liberia, the WHO reports three new cases, two of them probable and one suspected.

August 9, 2014

On August 8 the WHO declares the outbreak to be a public health emergency of international concern.Two medical staff who treated US citizen Patrick Sawyer die from the disease, and in a sign of the growing concern at the spread of the disease, a US company that makes an experimental drug for virus said it sends all its available supplies to west Africa. The biomedical collaboration between US and Canadian researchers involves a drug that is manufactured in tobacco leaves and is hard to produce on a large scale.Despite never previously being tested on humans, ZMapp was used to successfully treat two North American patients who had become infected when working to halt the spread of the epidemic.In response the Liberian government requested any supplies of the drug, and the head of the World Health Organisation, authorised the dispatch to Liberia to support the treatment of affected doctors

August 16, 2014

The outbreak continues to spread and on August 19 Cameroon closes its borders with Nigeria in an effort to halt the spread of the virus.The WHO said it was working with the UN's World Food Program to ensure food delivery to 1 million people living in Ebola quarantine zones cordoned off by local security forces in a border zone of Guinea, Liberia and Sierra Leone.It has told countries affected by the outbreak to screen people departing at airports, seaports and major land border points and stop any with signs of the virus.

August 31, 2014

Fears over the scale of the epidemic are again heightened as the disease begins to be reported in major cities in the region.The WHO says the outbreak could infect 20,000 people before it runs its course, warning that the real infection rates are likely two to four times higher than what is being reported.Reports are also firming up that a mysterious disease that has killed 13 people in the Democratic Republic of Congo could be an unrelated outbreak, which if proven will further stretch regional health resources.

September 14, 2014

The West African outbreak has spread to Senegal, where there have been three reported cases but no deaths.As of September 6, there had been 4,293 cases of Ebola in the West African outbreak, resulting in 2,296 deaths (a mortality rate greater than 50 per cent).On September 11, the Democratic Republic of Congo confirms a mysterious disease which has killed 13 people in the north of the country is Ebola.Health Minister Felix Numbi says the deaths occurred in an isolated area and the disease appeared to be a different strain from the one affecting West Africa.

September 14, 2014

The US announces it will send 3,000 troops to West Africa to try halt the spread of the deadly virus, with US president Barack Obama warning the epidemic in West Africa is "spiralling out of control".Mr Obama's plan calls for sending troops, including engineers and medical personnel, to build 17 treatment centres with 100 beds each, train thousands of healthcare workers and establish a military control centre for coordination of the relief effort, but service members will not be responsible for direct patient care.The move comes as the WHO announces it is establishing a global response coalition to respond to the crisis, and officials say the numbers of dead and infected have doubled in the last fortnight.

September 28, 2014

A man who became infected with Ebola in Liberia and travelled to Texas becomes the first confirmed case of the deadly virus in the United States.Dallas County officials announced 12 to 18 people had direct contact with the patient, Liberian national Thomas Eric Duncan, who flew to Texas from Liberia via Brussels and Washington on September 20.In Sierra Leone the government quarantined more than 1 million people in an attempt to halt the outbreak, with only health workers and people delivering essential items allowed to move within the new quarantine zonesAnd the World Health Organisation warned the number of cases could triple unless more was done.

October 1, 2014

Scientists predict there is a 75 per cent chance the Ebola virus could reach Europe before the end of the month.Virus experts used Ebola spread patterns and airline traffic data to predict that by October 24, there is a 75 per cent chance Ebola will have spread to France and a 50 per cent likelihood it will have been imported into Britain.In recent weeks it has spread to Nigeria, Senegal and the United States - where the first case was diagnosed in a man who flew in from Liberia - by unwitting travellers carrying the virus.

October 5, 2014

Scientists' predictions the virus could reach Europe are borne out: a Spanish nurse is diagnosed with Ebola in the first case of contagion outside of Africa.The nurse, Teresa Romero, had been treating two Spanish missionaries who contracted the disease in West Africa before she fell ill.She is in a stable condition in hospital in Madrid, and several other people, including Ms Romero's husband Javier Limon, have been hospitalised in an attempt to stem the spread of infection.The World Health Organisation (WHO) said the spread of the virus outside Africa was now "unavoidable", although it added the chances of a full-blown global outbreak were slim.

October 24, 2014

The number of people infected with Ebola passes 10,000, jumping to 12,008, as information emerges that Ebola is spreading up to nine times faster in parts of Sierra Leone than it was two months ago.

November 4, 2014

On November 5, Australian Prime Minister Tony Abbott confirms the Australian Government will help staff an Ebola treatment centre in Sierra Leone.The Government is committing up to $20 million to the 100-bed treatment centre, which is being built by the United Kingdom.The Prime Minister says the Government will not deploy health workers but will contract private Australian company Aspen to staff the centre.The United Kingdom has agreed to treat any Australian working in the region.

December 29, 2014

As of December 29, 20,206 people had contracted Ebola in the West African outbreak, resulting in 7,905 deaths (a mortality rate of 39 per cent).